Conventionally, pre-ignition combustion events, which, under operating conditions with high load or high EGR (EGR: Exhaust Gas Recirculation, which is herein used to encompass external EGR and internal EGR), engines that have high compression ratios may be prone to, become an issue on many occasions.
A tendency that pre-ignition combustion events take place results from higher compression ratios due to an increase in an intake of combustible charge entering a cylinder during operation with high load than compression ratios during driving with low load and a rise in cylinder temperature due to an increase in recirculation of high temperature exhaust gases into the cylinder caused by an increase in an amount of EGR. The pre-ignition combustion events may cause very high in-cylinder pressures, and may result in serious damages to a piston in the cylinder and a cylinder head.
In one example, JP Patent No. 3669175 discloses technology on EGR to address the issue. According to this technology, when a level of the pre-ignition combustion events is greater than or equal to a predetermined level, a valve overlap target in an amount of intake and exhaust valve overlap is reduced compared with that in the ordinary use. According to the technology disclosed by JP Patent No. 3669175, a tendency that exhaust system burned gases may be pushed through an engine cylinder during intake and exhaust valve periods is restrained to reduce an amount of high temperature unburned residual gases, lowering gas temperature in the cylinder, thereby avoiding the pre-ignition combustion events.
Incidentally, there is one approach for early detection of pre-ignition (or prediction of pre-ignition) based on detection of knocking (e.g., a defined number of subsequent occurrences of knocking). Among early combustion events, some are derived from a rise in temperature of cylinder inner walls (or a piston crown, a cylinder head combustion chamber) due to knocking while others without any indication of knocking under certain temperature conditions (intake temperature, exhaust gas temperature, engine coolant temperature and etc). Thus, it is difficult to always predict pre-ignition based on detection of knocking.
As another approach for detection of pre-ignition (or prediction of pre-ignition), pre-ignition is detected by measuring ion currents. In any case, prediction of pre-ignition is needed because it is likely that pre-ignition combustion events can result in serious damages to engines (pistons and etc.).
As one of various pre-ignition mitigating steps, by injecting fuel into a cylinder of a PFI (Port Fuel Injection) type engine during intake stroke when an intake valve opens, the vaporization heat of fuel is utilized to achieve cylinder charge cooling, which reduces likelihood of cylinder pre-ignition combustion events. However, the injection of fuel during intake stroke into the cylinder may cause HC (hydrocarbon) and CO (carbon monoxide), which are toxic unburned components, to increase. Moreover, the injection of fuel into a cylinder during intake stroke may result in admission of liquid fuel into the cylinder, thereby increasing likelihood of occurrence of soot. It is for this reason that avoiding always injecting fuel during intake stroke is desirable. Frequently injecting fuel into the cylinder during intake stroke may cause the injected fuel to adhere to the cylinder liner, thereby increasing likelihood of lubricant oil dilution by the adhered fuel.
The injection of fuel during exhaust stroke may reduce likelihood of the occurrence of HC, CO, soot and so forth because of the atomization of the injected fuel within an intake port before the injected fuel is combusted in a cylinder.